…rce_address. r=gw

This user_data field is currently only used as an address for some shader-specific resource in the gpu cache. We can always rename it back to something generic if we ever need to use the bits differently in other shaders in the future.

Depends on D51884

Differential Revision: https://phabricator.services.mozilla.com/D51885

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/cd13f04835f4d9ebdfccd14b5f4afe6a8a8a2dbe